a. Field of the Invention
The present invention relates to an improved automatic valve for controlling the vacuum in an enclosed container such as, for example, a suction box as used on a paper making machine.
B. Description of Prior Art
In the operation of the conventional paper making machine, the paper is formed by flowing a thin suspension of stock fibers in water from a head-box slice onto the upper surface of a moving endless screen belt which rotates over a breast roll at one end of the forming section of the machine, and a couch roll at the other end. As the belt travels in contact with table rolls or foils and suction boxes located between the breast and couch rolls, water is withdrawn from the stock through the belt leaving a thin formation of self-supporting, matted fibers on the upper surface of the belt. The sheet of formed fibers is lifted off the belt at the couch roll at the downstream end of the forming section and the belt, after travelling around the couch roll, is returned through a series of return rolls to the upstream end of the forming section where it travels around the breast roll and again passes under the slice to complete the cycle.
Most of the water is drained from the stock by the table rolls or foils. The stock is then further dewatered as it passes over the suction boxes under the belt. Suction boxes extend transversely of the forming section of the machine. Generally, a series of four or more suction boxes is positioned under the upper run of the belt towards the couch roll end of the forming section. Each box is constructed in the form of a long enclosed container approximately one foot or more in width and in depth. The cover of the suction box that contacts the underside of the screen belt is made of some durable material that resists wear and in turn provides support for the belt with a minimum of friction. The cover may be end-of-grain maple wood or in more recent installations some kind of hard wearing ceramic material. The cover is provided with an array of circular openings, or in some cases, slots that permit water to be drained by suction through the belt and into the interior of the suction box. In operation, a vacuum is maintained in the box by withdrawing air through an opening at one end. Water drained from the stock collects in the box and is evacuated through downspouts, or downcomers as they are called, through self-sealing trap fixtures. The water builds up to the required depth in the downcomer equivalent to the amount of vacuum that is to be maintained in the box. In the case of the suction boxes with which this invention is concerned, the vacuum may be high, up to 10 inches of mercury or low, less than 25 inches of water. The invention has been found to work particularly well in the low vacuum ranges.
Vacuum is maintained in a header pipe at the paper machine which is connected to each of the series of suction boxes by an exhaust pipe at the rear side of the machine. The vacuum in the header pipe is generally slightly greater than required and is controlled in each suction box either by a simple hand operated bleed valve at the front side of the machine or a mechanically complicated automatic valve with a vacuum sensor system, to control and maintain constant vacuum at the exhaust pipe leading from a suction box to the vacuum header pipe.
In the case of the hand operated bleed valve, the valve is opened sufficiently to allow the passage of air into the suction box to reduce vacuum therein to the desired amount as registered on a vacuum gauge. This hand operated bleed valve system needs constant surveillance because many variables will necessitate adjustment to maintain constant vacuum within the suction box. Variations in the vacuum source, in the pulp stock such as fiber content or fillers, in slice opening or in speed of the machine will all influence suction box vacuum. Changes in table configuration such as removal or addition of foils will also require adjustment to be made to suction box vacuum.
The conventional automatic valves are mechanically complicated and, being on the exhaust side of the suction box, they are frequently fouled by fibers of paper stock being sucked into the box, so that they may become inaccurate or non-functional.
Conventional poppet or ball type check valves are not generally adjustable. Known vacuum relief valves that are adjustable are either provided with a spring loaded diaphragm or spring loaded ball and seat arrangement. Such valves open against the loading set on a compressed spring and as the valve opens wider the spring is compressed to a greater extent so that, unless it is relatively long, the loading increases. Equilibrium may therefore be reached at a point where insufficient volume of air can pass the valve to maintain the desired vacuum in the suction box and the valve becomes difficult to set. Further, such valves are subject to inaccuracies because of corrosion or mechanical failure.